In Sher, U.S. Pat. No. 4,152,597, there is disclosed an MIS photocapacitive detector including a superionic relatively thin electrical insulating layer (preferably lanthanum trifluoride) that is formed on one face of an intrinsic semiconductor. An opposite face of the intrinsic semiconductor is connected to an electrode. A second electrode is formed on the superionic electrically insulating layer. The second electrode and superionic insulating layer are transparent to optical energy that is cyclically modulated and which is incident on the intrinsic semiconductor. The image incident on the intrinsic semiconductor has wavelengths with photon energy greater than the band gap of the intrinsic semiconductor to modulate the thickness of a depletion layer in the semiconductor. Thereby, the capacitance between the electrodes is modulated.
In Miller et al, U.S. Pat. No. 4,331,873, there is disclosed a photocapacitive image converter employing some of the principles involved in the Sher photocapacitive detector. In the Miller et al converter, a back face of a semiconductor substrate is completely covered by an electrode making ohmic contact with the substrate. On the opposite, front face of the substrate is deposited an epitaxial layer of a semi-intrinsic semiconductor, i.e., a semiconductor having a low amount of dopant and a low carrier concentration. The epitaxial layer is covered by a composite insulator formed of a thin film of silicon dioxide, in turn covered by a thin film of the superionic insulator. On the insulator is a first set of plural, spaced stripe electrodes extending in an X direction of a Cartesian coordinate system. The first electrodes are covered by a composite insulator, similar to the one covering the epitaxial layer. On the second composite insulator is a second set of plural separate stripe electrodes extending in the Y direction. The electrodes, insulating layers and epitaxial layer are transparent so that the thickness of a depletion layer in the semiconductor substrate is modulated by the image. In an attempt to minimize cross talk between the separate electrode stripes, and minimize the separation between adjacent stripes while providing a large number of stripes per unit area, a doped region is implanted in the epitaxial semiconductor layer in gaps between the stripes extending in the X direction and the stripes extending in the Y direction.
The converter is sealed in an appropriate component package having a transparent window over the front face of the substrate to allow light to be incident thereon. The package is pressurized with an inert gas to prevent fogging and corrosion.
Each of the electrode stripes extending in the Y direction is connected to one terminal of a multifrequency AC source, such that a different frequency is applied to each of the stripes extending in the Y direction. Each of the stripes extending in the X direction is connected to one terminal of a separate spectrum analyzing circuit. The remaining, usually ground terminals of the AC sources and spectrum analyzers are connected to the electrode on the back face of the detector array. The optical image incident on the semiconductor substrate is cyclically modulated.
There is thus formed a pixel at each intersection where the X and Y extending electrode stripes are in registration. In response to the frequency of the AC source coupled to each pixel and the image variations incident on each pixel, there is a voltage coupled to a processing circuit connected to each X electrode stripe. The voltage supplied to each processing circuit has an amplititude indicative of the light intensity on each pixel coupled to the X stripe. The frequency coupled by each pixel to the X stripe corresponding with the pixel is equal to the frequency of the AC source exciting the pixel as changed by the image frequency. The processing circuits separate the different frequency components of the plural pixels coupled to the same X stripe to enable the image to be reconstituted.
The prior art device described in the Miller et al patent has relatively low sensitivity and is susceptible to cross talk between the signals coupled to the output electrode stripes. The device of the Miller et al patent has relatively low sensitivity because the capacitance variation of each pixel coupled to an X output stripe is only a small percentage of the total capacitance coupled to the X output stripe. The total capacitance coupled to each output stripe subsists between one of the X output stripes and the grounded electrode covering the back face of the substrate. The signal coupled to a processing circuit from a single pixel is significantly attenuated by a factor equal to the relatively small ratio of the area of a single pixel to the area of the X electrode output stripe coupled to the pixel.
We have experimentally found that appreciable cross talk also subsists between the pixels associated with the adjacent electrode stripes. The attempt to prevent such cross talk by use of ion implanted heavily doped regions which form a grid of channel stops in the gaps between the stripes extending in the X and Y directions (as discussed in the Miller et al patent) does not provide adequate separation for very closely spaced stripe electrodes.
It is, accordingly, an object of the present invention to provide a new and improved photocapacitive detector.
Another object of the invention is to provide a new and improved photocapacitive detector array having improved sensitivity.
A further object of the invention is to provide a new and improved photocapacitive detector array having low cross talk between pixels formed between adjacent electrode stripes.
Still another object of the present invention is to provide a new and improved photocapacitive detector array having high sensitivity, low cross talk, and high density elements which result in high resolution.